Wet-process phosphoric acid

Wet-process phosphoric acid denotes the manufacture of phosphoric acid (H3PO4) through the decomposition of phosphate minerals by means of a mineral acid. Unlike the thermal route, which begins with elemental phosphorus obtained via electrothermal reduction of phosphate rock, the wet process proceeds directly to acid attack of the ore. Within the European Union, this production route accounts for roughly 95% of total phosphoric acid output^[1]. The preference for solvent-extraction purification over the thermal route stems from energy costs^[1]. Sulphuric acid serves as the favoured attacking agent owing to its production volume; nitric acid and hydrochloric acid find only marginal use^[1].

Main process variants

Attack by H2SO4 gives rise to several technologies differentiated chiefly by the crystalline form of the co-produced calcium sulphate. The dihydrate (DH) process crystallises gypsum as dihydrate (CaSO4·2H2O) and represents the most widespread technique. It yields a relatively dilute acid solution, assaying 26–32% P2O5, and achieves a P2O5 recovery of 94–96%^[2]. The hemihydrate (HH) process forms calcium sulphate hemihydrate (CaSO4·0.5H2O). Recrystallisation processes combine multiple stages to improve recovery. Double-filtration or repulping techniques enable reduction of P2O5 losses contained in the filtered gypsum^[3]. A modified dihydrate process, with double attack and extended residence time, attains an average recovery above 97.5%^[2].

Consumptions and recoveries

The manufacture of one tonne of P2O5 by the wet route generally consumes 2.6–3.5 tonnes of phosphate rock depending on ore composition^[4]. Sulphuric acid requirement falls within the range of 2.4–2.9 tonnes per tonne of P2O5^[4]. For process water, a given installation uses 3.6–4 cubic metres per tonne of P2O5, excluding scrubber water; other configurations consume 4–7 cubic metres depending on the concentration of weak acid obtained at the filter^[4]. Existing installations should target a P2O5 recovery of 94.0–98.5%, whilst new units must reach 98.0% at minimum, preferably through recrystallisation with double filtration^[3]. According to the European reference document, the main commercial concentration of the acid stands at 52–54% P2O5^[5].

Co-products and environment

The principal by-product of the process is phosphogypsum, a calcium sulphate arising from the reaction, which occurs as either dihydrate or hemihydrate depending on the technology selected. This material, still containing traces of P2O5 and metallic impurities from the rock, is subject to dedicated storage. Recycling of drainage waters from the phosphogypsum stack to the process is common practice in the industry^[2]. Atmospheric P2O5 emissions are controlled by entrainment separators, notably on flash coolers and vacuum evaporators, as well as by liquid-ring pumps with seal-liquid recycle^[3]. Phosphate rock selection and prior grinding directly influence process performance and the quality of acid obtained^[3].

Integration in the phosphate chain

Wet-process phosphoric acid constitutes the essential intermediate for the main phosphate fertilisers. DAP (18% N, 46% P2O5), MAP (11% N, 52% P2O5) and TSP (46% P2O5) all derive from it, through reaction with ammonia or phosphate rock^[6]. In 2023, world phosphate rock production reached 216 Mt, of which 30 Mt was exported; phosphoric acid production hit 85 Mt^[6]. In the United States, over 95% of extracted phosphate rock feeds wet-process phosphoric acid and superphosphoric acid production, themselves converted into granular or liquid ammonium phosphate fertilisers and animal feed supplements; roughly 25% of acid produced is exported as DAP, MAP, merchant acid or other phosphate products^[7]. Solvent-extraction purified acid serves in the manufacture of food-grade phosphates and industrial phosphates (sodium, potassium, calcium, ammonium salts), although this route is now poorly represented in Europe^[8].